Magma Metallogenic System Of Machangqing Copper-molybdenum-gold Polymetallic Ore Deposit,Yunnan Province

The Machangqing Cu-Mo-Au ore deposit is one of the typical deposits related to alkaline-rich porphyry of Himalaya epoch in the Jinsha River-Ailaoshan structural belt.It is generally belonging to porphyry type Cu-Mo-Au deposit,Baoxingchang rock is believed to be the porphyry and directly mineralized the ore deposit.However,plenty of evidence contraryed to this,it is necessary to rediscover the origin of Machangqing ore deposit.Field observation and laboratory research suggests that Baoxingchang rock and other rocks exposed large area(such as Shuanmacao rock)are porphyritic biotite adamellite,matrix is phanerocrystalline texture,thus they are not porphyry rock and certainly not the ore-forming intrusion.Outcrops and exploration section also contirm that Baoxingchang rock is not the ore-forming intrusion,because Baoxingchang rock is a rootless sheet intrusion,the mineralization is limited to the lower part of the rock and the main mineralization type is the vein mineralization filling the fissure.Instead,the mineralization of the small porphyry rocks be shield ender the Baoxingchang rock is better,disseminated mineralization and dense disseminated mineralization show the characteristics of ore-forming intrusions.Accordingly,the author considers that Baoxingchang rock is a denudation residue by Machangqing batholith and the ore bearing porphyry rocks are the ore-forming intrusions.According to zircon U-Pb LA-ICP-MS dating,the forming time of Baoxingchang and Shuanmacao porphyritic biotite adamellite is respective for 37.37±0.18 Ma and 37.57±0.42 Ma;the quartz syenite,quartz syenite porphyry and granite porphyry be shield under the batholith formed respectively for 34.71 ±0.14 Ma,35.34±0.55 Ma,and 34.5±0.16 Ma,it illustrated two episodes of magmatic intrusion activities in this area.As we know,the invasion depth of granitic batholith is general 10-12 km,the formation depth of porphyry ore deposits is less than 3 km,so it can be inferred that the region of Machangqing rapidly uplifted and denudated during the period of 2 Ma.The average formation depth of Machangqing batholith is 11 km according to estimated value of hornblende aluminum thermobarometer and consolidation depth about the porphyry rocks is near the ground surface confirmed the view,denudation rate is about 3.5?5 mm/yr.The research for the relationship between time and room of Machangqing magma metallogenic system indicates that,the magmatism and metallization in this region is rigorously controlled by the construction evolution of Jinsha River-Ailaoshan belt,existence of porphyritic biotite adamellite hinted there was a deep magma chamber in the crust of Jinsha River-Ailaoshan belt,lithosphere delamination made the injection of hyperthermal mantle-derived magma during 37 Ma to 35 Ma,plenty of ore-forming fluid activated the frozen magma chamber at the same time and triggered the mineralization in this region.